Induction coupled plasma generators



Jan 3, 1967 J. HEDGER INDUCTION COUPLED PLASMA GENERATORS Filed June 17,1963 United States Patent 3,296,410 INDUCTION COUPLED PLASMA GENERATORSHarry John Hedger, Chilton, Didcot, England, assignor to United KingdomAtomic Energy Authority, London, England Filed June 17, 1963, Ser. No.288,411 Claims priority, application Great Britain, June 20, 1962,23,703/ 62 7 Claims. (Cl. 219-121) The present invention relates toinduction coupled plasma generators.

A known induction coupled plasma generator operates by passing a streamof gas, suitably confined in a tube, through an induction coil which isarranged to be connected to the output of -a radio frequency generator.Hereinafter such plasma generator will be referred to as a plasmagenerator of the type described. The gas may be at a pressure of theorder of one atmosphere and in a typical case the generator may supplyabout 6 kw. of power at a frequency of 2.4 rnc./s. (which will sufliceto form and maintain a plasma in a gas such as argon flowing through atube of 1" internal diameter. A use for such a plasma generator isdescribed in our French Patent 1,328,697.

In order to initiate the plasma discharge in an arrangement as describedin said patent, it is necessary to insert a mass of graphite or somesimilar refractory conducting material within the field of the coil andthen to withdraw it when the plasma discharge forms. It appears that themass of graphite is heated by the coil and generates a sufiicient numberof ions under the voltage gradients in the coil field to initiate theplasma discharge which then becomes self-sustaining due to the very hightemperature. It is inconvenient for many purposes to have to withdrawthe graphite from the vicinity of the plasma discharge after thedischarge has been initiated and consequently it is an object of thepresent invention to provide an alternative means for initiating plasmadischarges of the above described induction coupled type.

According to the present invention there is provided a method for theinitiation of a plasma discharge in a plasma generator of the typedescribed comprising moice mentarily connecting an electricallyconducting body located upstream of the induction coil to the end of theinduction coil which is furthest removed from said conducting body.

According to a further aspect of the present invention there is alsoprovided a plasma generator of the type described with an electricallyconducting body located within the generator upstream of the inductioncoil (of the said plasma generator), and means to connect saidconducting body to the end of the induction coil which is furthestremove-d from said conducting body.

The conducting body may be arranged to be connected directly to said endof the induction coil or by way of a portion of a second coil which isconnected in series with the induction coil.

The term upstream refers to the direction of gas flow in the tube withinthe induction coil. However, it should be appreciated that the plasmadischarge generators with which the present invention is concernedoperate at substantially atmospheric pressure, and thus it is normalthat the tube within which they are generated should be substantiallyvertical, for otherwise, the very high temperature involved might causemelting of the tube, and consequently the induction coil which normallysurrounds the tube also has its axis generally vertical. Consequentlythe electrically conducting body which is located upstream of theinduction coil and which may, for example, be of tungsten or carbon, maybe located vertically above the induction coil. The advantage of thepresent invention is that the conducting body may be locatedsufficiently far away from the induction coil to prevent it beingnecessary to remove the conducting body when the plasma has beeninitiated. It is thought probable that a discharge takes place betweenthe electrically conducting body and the upper part of the coil somewhatin the manner of a capacitative discharge and that this dischargeprovide a sufliciency of ions to initiate the induction coupled plasma.

It has been found moreover that the present invention permits theinitiating and maintaining of the plasma discharge in larger tubes thanhas hitherto been possible with the same radio frequency generator andthe following figures illustrate the result of comparative experiments.

Minimum striking requirements of induction coupled plasma OrdinaryMethod Internal Dra. and type of Torch Before Starting Alter Starting V1V2 C V1 V2 C P 1 aircooled- 4. 3 4. 2 1. 0 1. 8 4. 0 1. 9 3. 6 1 watercooled 8. 2 7.0 1. 2 2.0 6. 8 4 19. 6 1% aircooled. Pulled overload onstriking 2% aircoolcd I Nlot starteld I New Method Internal Dia. andtype of Torch Before Starting After Starting V1 V; 0 V1 V2 C P 1aircooled 3. 2 3. 2 0.7 1.8 3.1 0. 98 0.9 1" 4. 8 4. 2 0.72 2. 5 4. 21.6 3. 6 1V aircooled.-- 3. 2 3. 22 0. 62 1. 5 3. 1 1. 1 1. 57 2%aircoolcd 2. 8 2. 6 0. 38 1. 2 2. 6 0. 35

where:

V =Peak kilovolts R.F. across coil using capicitance and resistancevoltage divider. Harmonics believed to be negligible from wave form.

V =D.C. kilovolts in power supply.

C=Total D.C. anode current in amps.

P=Approximate additional power in kilowatts supplied to oscillatorvalves due to presence of plasma.

A further feature of the present invention is that the conducting membermay be connected to a tapping on a further coil connected in series withthe induction coil. The importance of this is that in many cases theinduction coil acts as the tank coil of the radio frequency generatorand the very high coupling in the induction coil when the plasmadischarge is present may cause difficulties in the operation of thegenerator. If the load on the generator is formed by two coils in seriesas suggested then without the arrangement of the present invention itbecomes difficult to initiate the plasma, due to {he lower voltage whichappears across the induction cor In order that the invention may morereadily be understood, two embodiments of the same will now be describedby way of example and with reference to the accompanying drawings inwhich:

FIG. 1 is a diagrammatic section showing the associated electricalcircuit using a single coil.

FIG. 2 is a diagrammatic representation of the apparatus using a splitcoil.

Referring firstly to FIG. 1 the plasma generator comprises a verticalsilica tube 1, which is widened out in a bell shape at its lower end.The tube 1 is surrounded for the greater part of its length by a waterjacket 2, having inflow and outflow pipes 3 and 4 respectively, thewater jacket 2 also being bell shaped to correspond to the shaping ofthe tube 1.

A coil 5 of hollow copper tubing surrounds the water jacket 2, the coil5 being placed immediately above the bell shaped lower end of the waterjacket 2. Although. only five turns are shown, a larger number of turnsis preferred, ten turns being a convenient number. The ends 6 and 7 ofthe coil 5 are connected by leads 8 and 9 to a suitable radio frequencygenerator 10. Also connected to the ends 6 and 7 of the coil 5 are waterinlet and outlet connections 11 and 12, so that cooling water may besupplied to the coil 2.

The top of the tube 1 has a plug 13 sealed thereto, this plug 13 havingan axial bore into which is sealed a brass sleeve 14 which terminates ashort distance above the coil 5 and which carries at its lower end atungsten probe 15. At its upper end, the sleeve 14 is connected througha switch 16 to the lower end 6 of the coil 5. A silica tube 17 passes coaxially through the sleeve 14, and terminates in the region of the upperend 7 of the coil 5.

The tube 17 is sealed to the sleeve 14 and may be connected to avibrating hopper (not shown) which contains, in powder form, thematerial which is to be treated within the plasma. The tube 17 isprovided with a side arm 18, through which a suitable gas, for example,argon, may be introduced and mixed with the powder, prior to its passinginto the plasma. The plug 13 also carries a gas inlet connection 19,through which the gas producing the plasma is passed. It may also beconsidered desirable to provide a gas inlet connection to the top of thetube 1 above the water jacket 2, the gas passing through the connection20 providing a blanket between the plasma and the tube 1.

In order to use the apparatus, the gas flow through the connections 18,19 and 20 is turned on and the flow of cooling water through the waterjacket 2 and coil 5 is started. The radio-frequency supply to the coil 5is then turned on and the plasma is initiated by momentarily closing theswitch 16. When the switch 16 is closed the probe 15 is electricallyconnected to the lower end 6 of the coil 5 and the plasma is initiatedvery quickly. Once the plasma has been set up, the connection betweenthe probe 15 and the coil 5 may be broken, and the switch 16 istherefore opened. The closing and opening of the switch 16 may be donemanually or it may be preferred to do it automatically, in which case aphoto cell may be focused on the region of the generator in which theplasma is to be induced, suoh photo cell. being electrically connectedto the switch 16 in such a manner that the increase in the intensity ofillumination of the cell caused by the initiation of the plasma causesthe switch 16 to be opened.

In FIG. 2, the generator is shown in a simple diagrammatic form, thedetailed construction being as for the generator hereinbefore described,corresponding parts being similarly numbered. The coil 5, which acts asthe tank coil of the radio frequency generator 10, is connected inseries to a second coil 21. The coils 5 and 21 will each have a givenimpedance which will depend on the nature of the coil itself. When aplasma is established in the coil 5 there will be a certain amount ofcoupling between the coil and the plasma and this will affect theelfective inductance of the coil 5. The inductance of the coil 21 willhowever remain constant and thus the total inductance will not change bythe same relative amount with the two coils present, than if only coil 5had been in the circuit. Since changes in the inductance will tend toaffect the frequency, the effect will be less with the coil 21 presentthan if it were absent. The coil 21 will therefore be effective to keepthe frequency more constant than would be the case with coil 5 alone,although it will not necessarily ensure that the frequency is constant.The coil 21 thus reduces variations in the frequency. The coil 21 mayconsist of 2 or 3 turns 9-12 inches diameter,. The conducting probe 15is connected through the switch 16 to a tapping on the coil 21, thetapping being in a position such as to obtain a suitable voltage for theinitiation of the plasma discharge. The method of operating thisapparatus is as hereinbefore described with reference to a plasmagenerator having only one coil.

Once the plasma has been initiated, either generator may be used in amanner similar to that described in our said patent.

It should be noted that the arrangement with a split coil isconveniently used in the larger diameter, for example about 2 inchesdiameter, generators, whilst the arrangement with a single coil may beapplied to the smaller generators.

I claim:

1. An induction coupled plasma generator comprising a tube, an inductioncoil arranged to be connected to the output of a radio frequencygenerator, said induction coil surrounding part of the said tube, meansto introduce a stream of gas into said tube, means having an outlet tointroduce a stream of particulate solid into the said tube in thevicinity of the induction coil, an electrically conducting body locatedwithin said tube and upstream of both the said outlet and said inductioncoil and means to connect said conducting body directly to the end ofthe induction coil which is furthest removed from said conducting body.

2. An induction coupled plasma generator according to claim 1 in whichsaid electrically conducting body is made of tungsten.

3. An induction coupled plasma generator according to claim 1 in whichsaid electrically conducting body is made of carbon.

4. An induction coupled plasma generator comprising a tube, an inductioncoil arranged to be connected to the output of a radio frequencygenerator, said induction coil surrounding part of said tube, a secondcoil connected in series with said induction coil, means having anoutlet to introduce a stream of gas into said tube in the vicinity ofthe induction coil, means to introduce a stream of particulate solidinto said tube, an electrically conducting body located within said tubeand upstream of both the said outlet and the said induction coil, andmeans to connect said conducting body through a portion of said secondcoil to the end of said induction coil which is furthest removed fromsaid conducting body.

5. An induction coupled plasma generator according to claim 4 in whichsaid electrically conducting body is made of tungsten.

6. An induction coupled plasma generator according to claim 4 in whichsaid electrically conducting body is made of carbon.

7. A method of initiating a plasma discharge in an induction coupledplasma generator which comprises the steps of passing a gas in which theplasma is to be established along the central axis of an induction coil,passing a high frequency alternating current through said inductioncoil, passing the high frequency alternating current through a secondcoil having a tapping connected in series References Cited by theExaminer UNITED STATES PATENTS 1/1960 Giannini et al. 3/1965 Curtis eta1 313-161 JOSEPH V. TRUHE, Primary Examiner.

DAVID J. GALVIN, Examiner

1. AN INDUCTION COUPLED PLASMA GENERATOR COMPRISING A TUBE, AN INDUCTIONCOIL ARRANGED TO BE CONNECTED TO THE OUTPUT OF A RADIO FREQUENCYGENERATOR, SAID INDUCTION COIL SURROUNDING PART OF THE SAID TUBE, MEANSTO INTRODUCE A STREAM OF GAS INTO SAID TUBE, MEANS HAVING AN OUTLET TOINTRODUCE A STREAM OF PARTICULATE SOLID INTO THE SAID TUBE IN THEVICINITY OF THE INDUCTION COIL, AN ELECTRICALLY CONDUCTING BODY LOCATEDWITHIN SAID TUBE AND UPSTREAM OF BOTH THE SAID OUTLET AND HIGH INDUCTIONCOIL AND MEANS FOR CONNECT SAID CONDUCTING BODY DIRECTLY TO THE END OFTHE INDUCTION COIL WHICH IS FURTHEST REMOVED FROM SAID CONDUCTING BODY.